This invention relates to a suspension control system for a vehicle provided at each wheel for controlling switching of suspension characteristics in response to roughness of a road surface. More particularly, the invention concerns a suspension control system for reliably switching an attenuation force accounting for a delay of a control system over the entire travel conditions.
In general, a suspension system is interposed between a vehicle body and a vehicle wheel with a spring and a shock absorber for absorbing an impact from a road surface by the spring and to attenuate a continuous vibration of its recoiling by the shock absorber, thereby moderating the impact. If the attenuation force of the shock absorber is large, rolling of the vehicle body is suppressed to improve stability of a vehicle attitude, but the shock absorber sensitively reacts to the roughness of the road surface, and hence a stiff riding feel is attained. On the other hand, if the attenuation force is small, the vibration input from the road surface decreases, and a soft riding comfort is obtained. Thus, recently, a semiactive suspension system has been developed in which an attenuation force variable apparatus is added to a shock absorber to control the attenuation force of suspension characteristics in response to good or bad road surface conditions, conditions of braking, starting, turning, etc., to allow steering stability and riding feel to be compatible.
In a control system for switching suspension characteristics dependent on the roughness of a road surface, a road surface sensor of an ultrasonic type for detecting the roughness of a road surface is provided at the front side of a vehicle body. In this case, the road surface sensor is desirably disposed as near as possible to a front wheel so as to decrease detecting error due to pitching of the vehicle body and to detect a road surface.
On the other hand, in the case where the road surface sensor is held fixedly, a period of time when a vehicle wheel actually reaches a road surface detecting position to be detected by the road surface sensor varies in accordance with vehicle speed. Further, a sensor system, a control system, and an actuator system respectively have intrinsic delay times. In the case where the delay time of the whole control system is considered, when the vehicle travels at a predetermined speed or faster, there is no timing margin for road surface information detected by the road surface sensor to be used for controlling switching of the suspension characteristics and to result in no optimum timing of controlling of switching. Therefore, it is always required to so control as to bring the timing for the vehicle wheel to arrive at a road surface detecting position into coincidence with the timing for switching the suspension characteristics by considering the delay time of the whole control system over the whole travel area of the vehicle.
Heretofore, regarding a control system for switching suspension characteristics dependent on the roughness of a road surface, there is prior art as disclosed, for example, in Japanese Patent Laid-Open Publication No. 182825/1991. In the prior art, a road roughness sensor is provided at a lower front end of a vehicle body, and suspension characteristics are switched based on an output of the sensor.
In this prior art, the road surface roughness sensor is fixedly provided at a vehicle body to always detect a road surface separately at a predetermined distance forwardly of its front wheel. Therefore, it cannot control switching of the suspension characteristics at an optimum timing due to the time delay of the whole control system when running at high speed.
If the attenuation force of the shock absorber is large in the suspension, vibration can be rapidly attenuated to decrease changes in the attitude of the vehicle to improve steering stability. On the other hand, if the attenuation force is small, the vibration is continued to improve riding feel. Thus, recently, an attenuation force variable apparatus has been added to a shock absorber to control the attenuation force of suspension characteristics in response to good and bad road surface conditions, states of braking, starting, turning, etc., to provide compatibility of steering stability and riding feel.
In a stable attitude control for changing suspension characteristics depending on roughness of a road surface, a road surface sensor of an ultrasonic type for detecting roughness of the road surface is provided at the front of a vehicle body. In this case, the road surface sensor is desirably disposed as near as possible to a front wheel so as to decrease detecting error due to pitching of the vehicle body and so positioned as to detect the road surface. In the control system, the signal from the road surface sensor is inputted, processed in the control system, and actuated for the actuator of the attenuation force variable apparatus of the suspension.
Therefore, a sensor system, a control system, and an actuator system respectively have delay times. In the case where the sensor mounted position near the front wheel and the delay time of the whole control system are considered, there is no timing margin for road surface information detected by the road surface sensor to be used for controlling switching of the suspension characteristics and therefore no optimum timing of controlling switching is achieved. Thus, it is required to so control as to bring the timing for the vehicle wheel to arrive at a road surface detecting position into coincidence with the timing for switching the suspension characteristics by considering the delay time of the whole control system in the wide vehicle speed range of the vehicle. When four-wheeled suspensions are simultaneously controlled, it cannot be applied in the case where the roughness of the road surface occurs in a short period. Thus, it is necessary to control the suspensions in coincidence with the timing for the front and rear wheels to arrive at the rough road surface.